1. Field of the Invention
This invention relates to tractors for use with a fifth wheel trailer and, more particularly, to a terminal tractor which is adapted for maneuvering a trailer in a confined area and on ramps or other irregular road surfaces.
2. Description of the Prior Art
With the development of containerization it has become common to transport containers by water or rail on semi-trailers driven aboard container ships or railway cars. After the destination terminal is reached the semi-trailers are driven off the vessel or railway car, normally to a distribution yard. Generally the trailers are designed for on-highway use with conventional fifth wheel trucks so that they can be loaded and unloaded at sites remote from the terminal. Space is normally at a premium at terminal distribution yards, and hence maneuvering space is limited in the yards as well as on the container ships. Various vehicles have been used to move the trailers in such confined quarters including fork lift trucks, straddle carriers and terminal tractors. In the past terminal tractors have basically been shortened versions of conventional fifth-wheel trucks and have had their fifth wheel provided with a hydraulic lift controllable by the operator from the truck cab so that the landing gear (front stand) of the semi-trailers does not have to be cranked up each time that the trailers are moved in the terminal and loaded or unloaded on and off the ships or railway cars.
It is difficult to park a semi-trailer against a wall or other trailer by use of a conventional terminal tractor, particularly if backing space is quite limited. In some instances such a maneuver cannot be accomplished without uncoupling the tractor, moving it to a more acute angle relative to the tractor, recoupling it, and then backing the tractor to push the front of the trailer laterally. Even if this uncoupling procedure is not necessary, it may be required to jockey the tractor and trailer back and forth several times while slewing the tractor wheels in order to achieve the parking objective. On some occasions the quarters are so confined on both sides of the desired parking slot that it is virtually impossible to maneuver the trailer into the slot with a conventional terminal tractor.
Difficulties are also experienced in confined quarters even with a skilled operator, in backing a terminal tractor into coupling relationship between the fifth wheel plate of the tractor and the king pin of the trailer even though the fifth wheel plate normally has a double-oscillation mount and has a V-shaped opening for guiding the fifth wheel components together.
This maneuvering difficulty encountered when using conventional terminal tractors and trailers is also experienced, often to even a greater degree, when special trailers are used. For example, when using "roll" trailers, as shown in U.S. Pat. No. 3,874,703, a rearwardly projecting gooseneck attachment to the fifth wheel must be maneuvered by operation of the tractor so as to seat in a receptacle presented at the front of the trailer. Precision maneuvering is required to jockey the rearwardly projecting end of the gooseneck into the receptacle, and frequently the operator finds it necessary to jockey the tractor forwardly and rearwardly several times before meeting with success. This is of course time consuming and adds significantly to operational costs.
Because conventional trailer landing gears were not designed for the stresses encountered aboard ship, particularly in rough sea conditions, in some shipping operations rigid auxiliary stands are provided of the type disclosed in U.S. Pat. No. 3,603,544. Such a stand has a fifth wheel plate which can engage the kingpin of the trailer and also has a kingpin which can engage the fifth wheel plate of a terminal tractor. The stand can thus be used to couple a tractor to a trailer and, when released from the tractor, can become the support for the front end of the trailer on a ship's deck. Such stands are a boxlike structure open at the front to receive the rear end portion of the truck frame with the forward portions of the lateral sides of the stand fitting between the frame and the rear wheels of the truck. This arrangement gives little tolerance between the truck frame and the lateral sides of the stand. Furthermore, the fifth wheel plate of the tractor, rather than being a double oscillating unit as is conventional, is required to be single oscillating about the lateral horizontal axis when such stands are to be used. Hence, significantly more perfect alinement between the longitudinal axes of the truck and trailer is required during the coupling operation than would be required if the stands were not in use. As a result, when using a standard terminal tractor, a skilled operator frequently finds it necessary to jockey the tractor forward and backward several times to achieve the alinement necessary for coupling the tractor to the kingpin of the stand, even in those instances where there are not confined quarters at the coupling site of the trailer.
As previously mentioned, the above-described auxiliary stands normally require use of a single oscillating fifth wheel plate. This frequently results in compounding the severe loads on the tractor frame and elevator mounts for the fifth wheel plate occuring when the tractor is pulling a loaded trailer in a turn at the head or foot of a ramp as, for example, encountered on container ships between the decks. Under such conditions the front and rear wheels cannot all gain ramp or deck contact at the same time without twisting of the frame or the elevating mechanism for the fifth wheel. The resulting stresses commonly cause permanent deformation or failure of parts of the tractor. In this regard, the rear axle suspension for conventional terminal tractors is not of much assistance in alleviating frame stresses under the described conditions of ramp travel because the rear axle suspension of a terminal tractor should be stiff so that when the fifth wheel is elevated to raise the trailer sufficiently for adequate ground clearance, it will not be necessary to first overcome a relatively large vertical travel represented by compression of the rear axle suspension system due to the trailer load.
Another difficulty with terminal tractors is maintaining proper weight distribution so that the front wheels are not lifted out of road contact when a heavy torque load is applied to the rear axle from the engine. To maintain a proper weight balance it has been necessary to load the front bumper of conventional terminal tractors with counterweights in the neighborhood of 1200 to 2500 pounds if the tractor was to have a reasonably short wheel base for relatively short radius turning. Even then, the terminal tractors in use have not had as short a turning radius as needed for the tight turns frequently encountered.
Normally when the container semi-trailers are coupled to an on-highway truck for transport to and from a terminal, the landing gear is cranked up for maximum road clearance. When the trailer is to be uncoupled at the terminal storage yard the truck operator should crank down the landing gear to ground level before driving the truck free of the trailer. The lowering of the landing gear is a burdensome manual operation and frequently the operator doesn't fully carry it out. As a consequence, when the truck is driven from beneath the forward end of the trailer the front of the trailer drops until the landing gear engages the ground. If the ground surface is not paved, the lower end of the landing gear will often sink into the ground, particularly under wet conditions, and if the ground surface is black-topped, occasionally the inpact of the dropping landing gear will drive it through the asphalt, particularly on a hot day, thereby further lowering the front of the trailer. When such a trailer is to be later coupled to a terminal tractor, the tractor operator has need for the fifth wheel plate on the tractor to be capable of being lowered well below the normal coupling level of about 48 inches in order to make it possible to engage the kingpin of the trailer when the tractor is backed into coupling position. The problem of filling this need on a compact terminal tractor is compounded by the fact that it is normally preferred to have the fifth wheel plate occupy a position directly above or slightly forward of the rear axle so that the weight of the trailer will not leverage the front axle of the tractor in the upward direction. Hence, there are acute space limitations for the elevating mechanism for the fifth wheel plate and for any mechanism provided to permit the front and rear axles to freely occupy different planes from one another when loaded. The resulting design engineering problems are particularly formidable if the terminal tractor is to also fill the long felt need of being more maneuverable in such a manner as to significantly ease the operations of coupling to trailers of the various types encountered and parking of the trailers in confined quarters.